This present invention relates to actively controlled suspension systems and, in particular, to a suspension system having an active mode and a passive mode.
Active suspension systems are known to those skilled in the art. In general, active suspension systems control a vehicle's "ride" and "handling". Active suspension systems control the vehicle's "ride" and "handling" by controlling and maintaining the vehicle's attitude, for example, elevation, roll and pitch. Active suspension systems sense the attitude of the car and, in response, provide power to or remove power from actuators to maintain and control the "ride" and "handling" of the vehicle. For example, Williams et al., U.S. Pat. No. 4,625,993, teaches of a conventional active suspension system intended to maintain a vehicle substantially stable in all planes of movement irrespective of any external forces acting upon the vehicle.
Active suspension systems, for example, in automobiles and the like, include a pressurized fluid source and suspension units. The pressurized fluid source, for example, a hydraulic pump, provides fluid power in the form of a pressurized working fluid to the suspension unit at each vehicle wheel. The suspension units dissipate and/or counteract undesirable external forces.
When there is a failure in the suspension system, for example, an interruption in fluid communication between the pump and the suspension units, the suspension units are unable to function in the active mode. If the suspension system is to provide any suspension at all after loss of the active mode, the suspension system will be required to function in a passive mode as a passive-like suspension system.
In addition to a loss of the active mode, an interruption in fluid communication between the pump and the suspension units can cause serious damage to various elements of the suspension system. The pump communicates with each of the suspension units through conduits, for example, a main conduit coupled to the pump and secondary conduits which "feed off" the main conduit and supply each suspension unit with the pressurized fluid. An interruption in fluid communication can result from a break in the main conduit which would lead to a complete loss of fluid in each of the suspension units. This would cause serious damage to various elements of the suspension units as well as the pump.
To prevent such damage and still provide suspension for the vehicle, the suspension units include protection and bypass valves. For example, active suspension systems like the system disclosed in West German patent DE 3638-574A employ a 3-port, two state, switching valve and a 4-port, two state, power valve in an entirely hydraulic system. These valves are configured such that if there is a loss of hydraulic pressure or if there is an electrical power failure, the hydraulic pump is disconnected from the suspension unit and a fluid restrictor is incorporated between the two chambers of the double-acting cylinder to provide resistance to fluid flow between the chambers. In this configuration, the suspension functions in a passive mode.
A suspension system like the system disclosed in Doi et al, U.S. Pat. No. 4,787,650, utilizes a hydropneumatic spring, hydraulic actuator and valve configuration to control the damping characteristics of the suspension system. The Doi et al system increases or decreases the damping characteristics of the suspension system according to the vehicle's current "ride" and "handling" characteristics measured by acceleration detecting devices and vehicle attitude sensing devices. The damping characteristics of the suspension system are adjusted by opening or closing electromagnetic valves, which provide resistance to fluid flow, in the fluid paths between the hydropneumatic spring and hydraulic actuator.
Suspension systems like the system disclosed in Doi et al are single mode suspension systems. These type systems are often designed for active or semi-active modes of operation. The Doi et al type system does not, on the occurrence of some type of suspension system failure, modify its mode of operation from an active or semi-active mode to a passive-type mode.
There exists a need for a hydropneumatic suspension system to provide a default operating characteristic similar to conventional passive suspension systems.